1. Field of the Invention
The present invention relates to a reticle set used in a double exposure process, and furthermore, a method of designing the reticle set, a method of monitoring exposure using the reticle set, a method of inspecting the reticle set, and a method of manufacturing a semiconductor device.
2. Description of the Related Art
In recent years, a multiple exposure overlapping a plurality of exposure processes for delineating a specific pattern has attracted attention as one of different types of micro-fabrication technologies. The performance of semiconductor devices greatly depends on the dimensions of the wiring pattern. Therefore, in a process using the multiple exposure, it is required to control precisely parameters for improving accuracy of the dimensions. In the case of using the multiple exposure, exposure conditions are usually determined for each exposure step, and each exposure step for a multiple exposure process is performed based on to conduct multiple exposure the determined exposure conditions.
In a photolithography process, a semiconductor circuit pattern is transferred by use of an aligner on a semiconductor substrate coated with a resist film. In forming a pattern by a reduction projection aligner, the resolution of the aligner is proportional to a wavelength λ of an exposure light and inversely proportional to a numerical aperture NA. Accordingly, responding to the demand of finer dimensions of semiconductor devices, the wavelength of the exposure light has been shortened, the NA of the projection lens has been increased, and process improvement associated therewith has been achieved. However, in order to achieve even finer dimensions of semiconductor devices required in recent years, it is more difficult to assure an exposure latitude and a depth of focus. Therefore, in order to effectively make use of the small exposure margin and improve accuracy of processed dimensions without causing a reduction in production yield, more accurate control of the exposure dose, i.e. the amount of exposure, and the focus is required.
As to the exposure dose control, an exposure dose monitoring method has been proposed. In this method, exposure is performed so as to have an inclined distribution of the exposure doses on a substrate, using a reticle set having patterns therein with dimension ratios of transparent portions to opaque portions continuously changed in one direction by a pitch that cannot resolve images on the substrate by a reduction projection aligner (refer to Japanese Patent Laid-Open Application No. 2000-310850). Furthermore, as to the focus control, a method has been proposed in which focus is monitored using focus monitor patterns having a rhombus feature, that shift the exposure light in phase from each other to show different characteristics of pattern dimension for defocus (refer to Japanese Patent Laid-Open Application No. 2001-100392).
As described above, in micro-fabrication it is important to control exposure conditions for photolithography with high accuracy in order to achieve processing accuracy and uniformity in pattern dimensions of semiconductor devices. Therefore, it is necessary to provide many types of monitor marks such as an alignment mark, a dimension monitor mark, an exposure dose monitor mark and a focus monitor mark in the exposure area so that a large amount of data can be acquired and analyzed. However, in a reticle set on which a high density semiconductor circuit pattern has been delineated, the monitor marks are disposed only in limited areas such as scribe lines because of restrictions on layout. Accordingly, in an actual semiconductor integrated circuit manufacturing process, it is difficult to acquire detailed information on positions and dimensions over the entire exposure area. As described above, currently used configuration of monitor marks are an obstacle to the improvement of accuracy in micro-fabrication.